2003 silverado only blows on 5

The problem could be the vacuum line from heater control to the engine. Check where it comes through firewall and be sure it is connected. If ok problem is likely heater control switch.

It sounds like your coolant may be low and the higher rpms are forcing more coolant through the water pump. And yes your thermostat may be weak and stuck open.

Blower concern is the resistor
heater concern is the drivers heat/cold door actuator.

needs the drivers temp door actuator replaced-it is on the bottom of the heater box under the right side dash-has a small plug with 5 terminals

Try resister on the bottom of the glove box.

Check to see if the duct hose to the defroster vents has come loose. If air is blowing out under the dash when you have the control set to defrost, then this is likely.

new reostat unit

The air vents are control by vacuum line and or linkage.

Listen for hiss or feel for vacuum leak under the dash,

Feel for broken linkage(s) that are close to the vents.


WORK BACKWARD if you have small hands to feel out where is the missing link.

check under the dash to see if the blend door is operating, this ian internal door between the heater core and the a/c evaporator. also check that the drain hose from the evaporator make sure its clear and water is coming out of it,otherwise the evaporator will freeze solid and therefore no cold air

THE MOTOR THE CONTROL THE DOOR FOR VENTS THE GEAR ARE PLASTIC. ON THIS TIME REMOVE THE MOTOR AND CHECK THE DOOR BY HEAND IF DOOR OK REPLACE THE MOTOR CONTROLER.